Radical polymerization methods conventionally require that the monomers to be polymerized, a chain transfer agent, a source of free radicals and, optionally, a catalyst are brought into contact in at least one solvent.
The principal objective of a polymerization method is to obtain a polymer having a molecular mass suited to the application for which it is intended. The present invention aims to obtain polymers having a molecular mass of less than 8000 g/mol, for example of about 6000 g/mol.
Various methods of radical polymerization exist.
Mention may be made first of methods employing organic solvents, such as secondary alcohols like isopropanol. These methods are unsatisfactory today because they generate volatile organic compounds (VOCs). On the one hand, these solvents must be removed at the end of the reaction, thus complicating the industrial polymer preparation process. On the other hand, these solvents are recognized as having very harmful effects on health and on the environment, such that their production is sought to be avoided. Lastly, even after purification (distillation), traces of solvent still remain in the polymer solution.
Other methods for synthesizing polyacrylic polymers take place in water and do not generate volatile organic compounds.
Among the various radical polymerization methods, mention may also be made of reversible addition-fragmentation chain transfer (RAFT)-type controlled radical polymerization for carrying out the living polymerization of a monomer. Such a method also makes it possible to obtain polymers having low polydispersity (polymolecularity) indices (PI), which makes them particularly effective for certain applications.
To implement a RAFT-type controlled radical polymerization, and thus to obtain a polymer of expected molecular mass having a good PI, it is important to add to the reaction medium an available amount of chain transfer agent, i.e., to employ an amount of chain transfer agent such that each chain to be polymerized is functionalized by a chain transfer agent. Moreover, it is important that this chain transfer agent is already available when polymerization is initiated, i.e., when the polymerization reactor is heated and radicals are generated. This implies that large amounts of chain transfer agent must be employed in a RAFT-type controlled radical polymerization method.
Despite all the advantages of RAFT polymerization, the use of such amounts of chain transfer agent have a certain number of disadvantages.
First, chain transfer agents prove to be expensive products, significantly increasing the cost of the polymer obtained.
Moreover, when sulfur-containing chain transfer agents as described in the documents WO 02/070571, WO 2005/095466 and WO 2006/024706 are used, it is noted that a fraction of these compounds will be broken down to free sulfur-containing by-products of the CS2 and H2S type and be found in the aqueous solution of the final polymer and in the run-off water of the process, and thus can have a negative impact on humans and on the environment. Moreover, the presence of these sulfur-containing by-products in the aqueous solution, during the use of the polymer, generates gaseous emissions harmful to humans.
Alternative RAFT-type methods of controlled radical polymerization exist. According to one, hydrogen peroxide is used as initiator and, for example, copper sulfate as catalyst and chain transfer agent. Nevertheless, to obtain a polymer having a molecular mass of less than 8000 g/mol, for example of about 6000 g/mol, it is necessary to use a large amount of catalyst, which generates a large amount of polluting by-products.
Alternately, thiolactic acid or another RSH mercaptan is used as an additional chain transfer agent, but, again, in order to obtain a polymer having a molecular mass of less than 8000 g/mol, for example of about 6000 g/mol, it is necessary to use large amounts of thiolactic acid or, more generally, transfer agent.
Still other methods rely on sodium hypophosphite, having the chemical formula NaPO2H2, as chain transfer and reduction-oxidation agent, in the presence of hydrogen peroxide or radical generator. The document GB 771 573 A1 notably describes one such method. It has the major disadvantage of requiring large amounts of sodium hypophosphite, a phosphorus fraction being found grafted in the polymer, another phosphorus fraction being found in the form of phosphate salts in the process water. This is, first, a disadvantage during the use of the polymer and, second, an environmental pollutant.